Most applications of TEA CO2 lasers in heterodyne laser radars require that the transmitter have a high degree of frequency stability. This ensures good Doppler resolution and maximises receiver sensitivity. However the environment within the device is far from benign with fast acoustic and electrical transients being present. Consequently the phenomena which govern the frequency stability of pulsed lasers are quite different from those operative in their cw counterparts. This review concentrates on the mechanisms of chirping within the output pulse; pulse to pulse frequency drift may be eliminated by frequency measurement and correction on successive pulses. Experimental evidence for laser-induced, plasma, acoustic, and anomalous dispersive effects is examined, and it is demonstrated that normally only the first two effects are of any significance. It emerges that good stability hinges on correct cavity design. The energy-dependent laser-induced frequency sweep falls dramatically as mode diameter is increased. Thus it is necessary to construct resonators with good selectivity for single mode operation while having a large spot size. Various approaches to this requirement are described including recently developed novel techniques. Finally the future for compact frequency-stable devices is investigated.
David V Willetts,
"How To Design Frequency Stable TEA CO2 Lasers", Proc. SPIE 0663, Laser Radar Technology and Applications I, (6 October 1986); doi: 10.1117/12.938665; https://doi.org/10.1117/12.938665